Aquifers are generally characterized within a subterranean formation as a water table at some level below the ground surface with a capillary fringe region of varying depth overlying the water table. A vadose zone or unsaturated area typically extends between the capillary fringe and the ground surface.
Aquifers which have become contaminated, such as by the presence of gasoline and other organic contaminants, are often remediated by removing the groundwater from the aquifer and stripping contaminants from the groundwater which is then returned to the aquifer substantially free of contaminants. Vacuum extraction is one way in which removal of groundwater may be accomplished.
A typical vacuum extraction system for groundwater removal uses a casing within the borehole which extends down into the water table a depth at least as great as the expected drawdown in the water table to ensure the presence of water within the casing for pumping. The casing contains perforations in the wall to promote flow communication between the formation and the casing so that liquids and gases from the formation may enter the casing. A drop-tube extends from the surface down to adjacent the lower end of the casing to define an annular area between the drop-tube and the casing.
The drop tube includes a lower end which is open to the formation below the water table and an upper end which is connected to a vacuum pump. The well is usually sealed at the surface as by a cement plug which is cast in-situ.
Application of a vacuum to the tube pulls water from the annular area into the lower end of the drop-tube for removal from the formation. Examples of such systems are described in Blake et al., "Vacuum Enhanced Hydrocarbon Recovery: A Case History" Proceedings of the NWWA Conference on Petroleum Hydrocarbons and Organic Chemicals in Groundwater--Prevention, Detection and restoration (Nov. 12-14, 1986), the disclosure of which is incorporated by reference.
Conventional vacuum extraction systems are of limited usefulness in applications where the depth to water exceeds about 33 feet since vacuum pumping cannot remove a column of water exceeding this height. Various configurations have been proposed to enable groundwater removal by vacuum from depths greater than 33 feet by creation of a two-phase flow, but these are limited in several respects, particularly in regards to start-up in that the systems are not "self-priming" and thus require complicated start-up procedures.
Accordingly it is an object of the present invention to provide an improved system for recovering groundwater.
Another object of the present invention is to provide an self-priming system for removing groundwater from a subterranean aquifer.
An additional object of the present invention is to provide a system of the character described which enables the removal of liquids with reduced vacuum as compared to conventional vacuum extraction wells.
It is another object of the present invention to provide a system of the character described which enables liquids to be removed from depths below about 33 feet.
Yet another object of the present invention is to provide a system of the character described which provides a flowing column of a liquid/gas mixture from start-up to avoid the need for priming.
Yet another object of the present invention is to provide a system of the character described which overcomes disadvantages of prior art vacuum extraction systems.
Still another object of the present invention is to provide a system of the character described which is uncomplicated in configuration and economical to operate.